Many different instruments have been designed for on-line sensing of web characteristics. Such equipment particularly related to paper includes on-line sensors for paper formation, fibre orientation, wire mark, etc.
One of the earlier developments for detecting formation is described in a paper "A Formation Tester Which Graphically Records Paper Structure" by Burkhard et al published in the Pulp and Paper Magazine of Canada, 60, No. 6, T319-T334 (June 1960) wherein it was emphasized that direction was an important factor that had to be considered in an instrument for measuring paper structure. This device utilizes an illuminator and a photo electric cell to sense the amount of light transmitted through the web and generates a transmitted light signal which is analyzed in a frequency spectrum analyzer. The light source and photo electric cell move axially of a cylinder on which the paper sample is mounted.
A more recent system is described in an article entitled `Development of an On-line Formation Tester to Determine Optimal Use of Retention Aids` by Landmark et al published in the Paper Trade Journal, Sept. 1984, pages 84 to 86. This system is based on the use of tester incorporating a laser and that was developed by the Norwegian Pulp and Paper Institute to measure the variations in basis weight. In particular for the instrument described by Landmark et al a specific laser was selected and illuminated a spot about one millimeter in diameter by directing light through the web, the light transmitted through the web was then projected onto a photo cell.
In U.S. Pat. No. 4,648,712 issued Mar. 10, 1987 to Bernholt a light source illuminates one side of the web and the radiation passing through the web is detected by a pair of detectors having fields of view along narrow strips which are perpendicular to each other. The detector outputs are processed to provide fibre orientation ratio, formation and basis weight measurements.
U.S. Pat. No. 4,760,271 issued July 26, 1988 to Bernholt describes a system, which like the above described Bernholt system, illuminates paper from one side and provides optical means on the opposite side for detecting the light transmitted through the paper. A second illuminator is provided on the same side of the paper as the detector and directs light onto the paper at an oblique angle to a line normal to the plane of the paper web and the reflected light emanating from the second source is also detected.
U.S. Pat. No. 4,644,174 issued Feb. 17, 1987 to Ouellette et al also describes directing a beam through the paper and utilizes a photo detector receiving the light transmitted through the paper in combination with a tuneable filter and a demodulator to produce a DC output reflecting size and distribution of fibers or flocs.
It is also known to measure the surface properties of paper by optical measurements. A recent technique for such optical measurement is described in a paper entitled `Optical Measurement Throws New Light on Paper Surface` by Hansuebasi and Morantz published in Paper Technology in Industry, Aug. 1987. This technique employs a scanning goniometer which varies the angle of incidence of impingement of a polarized light and studies the angular reflectance and attempts to correlate printability with surface non-uniformity as determined by the equipment.
Equipment has also been proposed for surface inspection for defects wherein the entire surface web is scanned to detect defects or faults in the paper and enhanced graphics are used to visually display a defect such as a small tear or the like in the paper. Such a device described in a paper entitled `Solid State Surface Inspection Equipment` by P. W. Loose published in June, 1987 Tappi Journal, pages 69 to 74. This system uses a line scan camera having a charged coupled device (CCD) and an electronic exposure control. The amount of light applied to the CCD of the camera is regulated and the basic scanning rate of the camera is synchronized with the web speed.
An on-line dirt counting systems is described in an article by Kemeny et al. entitled `On-line Automatic Pulp Dirt Count Measurement` Tappi Conference proceedings, 1987, pages 2124 inclusive. The particular dirt counter described in this publication utilizes the CCD camera focused onto an illuminated surface of the pulp so that the light saturates the scanned segment of the pulp surface to eliminate any shadows which may tend to appear due to the roughness of the pulp surface. This device determines the dirt count by correlating the signal strength with a Base Line following threshold which was adjusted so that the system tracks around the normal product fluctuations and detects primarily sharp edge defects that generate a signal sufficiently high to trigger the system.